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An OrganicЦInorganic Hybrid Electrolyte Derived from Self-Assembly of a Poly(Ethylene Oxide)ЦPoly(Propylene Oxide)ЦPoly(Ethylene Oxide) Triblock Copolymer.

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Zuschriften
Conducting Materials
An Organic–Inorganic Hybrid Electrolyte
Derived from Self-Assembly of a Poly(Ethylene
Oxide)–Poly(Propylene Oxide)–Poly(Ethylene
Oxide) Triblock Copolymer**
Hsien-Ming Kao* and Chien-Liang Chen
Intensive studies have focused on the development of new
types of solid polymer electrolytes (SPEs) which combine
high mechanical strength with high ionic conductivity, as
needed for applications in, for example, solid-state lithium
polymer batteries.[1, 2] While poly(ethylene oxide) (PEO),
chains act as solvents for lithium salts, practical use of PEObased electrolytes is often hindered by their low conductivity
at room temperature (ca. 107–108 S cm1) and poor mechanical properties. Moreover, PEO-based electrolytes are prone
to crystallization. To alleviate these drawbacks of polyetherbased electrolytes, considerable effort is being devoted to
synthesizing new organic–inorganic hybrid materials by sol–
gel routes, which usually involve hydrolysis and condensation
of alkoxysilanes. A variety of organic–inorganic hybrids with
covalent bonds or only weak physical bonds between the
inorganic and organic phases have been proposed.[3–7] These
solid polymer electrolytes, so-called ormolytes (organically
modified electrolytes), combine the solvating power of the
ether units with an inorganic network, which provides
simultaneously an amorphous structure and good thermal,
mechanical, and chemical stability. However, miscibility
between the organic and inorganic entities is a major concern,
but this can be overcome by the use of functionalized
alkoxysilanes.
Block copolymers can be regarded as macromolecular
analogues of low-molecular-weight surfactants, and they can
self-assemble under certain conditions to give a variety of
nanoscale morphologies.[8] In particular, PEO-PPO-PEO
triblock copolymers (PPO = poly(propylene oxide)) consisting of two dissimilar moieties, that is, hydrophilic EO blocks
and hydrophobic PO blocks, were recently used as structuredirecting agents to prepare ordered mesoporous silica by selfassembly in acidic medium.[9] Herein we report the design and
synthesis of a new organic–inorganic hybrid electrolyte
derived from the self-assembly of a PEO-PPO-PEO triblock
copolymer by co-condensation of (3-glydicyloxypropyl)trimethoxysilane (GLYMO) and tetraethoxysilane (TEOS). We
[*] Prof. H.-M. Kao, C.-L. Chen
Department of Chemistry, National Central University
Chung-Li, Taiwan 32054 (Republic of China)
Fax: (+ 886) 342-27664
E-mail: hmkao@cc.ncu.edu.tw
[**] Financial support of this work by the National Science Council of
Taiwan is gratefully acknowledged. The authors thank Dr. U-Ser
Jeng, Prof. H. P. Lin, Mr. C. Y. Tang, and Ms. R. R. Wu for their
contributions to SAXS, TEM, and NMR experiments.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
998
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/ange.200352243
Angew. Chem. 2004, 116, 998 –998
Angewandte
Chemie
used commercially available Pluronic F127 triblock copolymer (EO106PO70EO106, Mw = 12 600, BASF) both as the
structure-directing amphiphilic surfactant and as the polymer
matrix. Since the EO and PO units of F127 are relatively
short, the mechanical properties of F127-based solid polymer
electrolyte are a major concern for its practical applications.
These properties can be greatly improved if the silica domain
is incorporated within the polymer matrix. The F127 triblock
copolymer was recently used for the self-assembly of ordered
cubic mesoporous silica SBA-16 in acidic medium.[9] We
therefore expected that F127 triblock copolymer would selfassemble on a functionalized silica network to generate a
nanocomposite film. Moreover, the epoxide functionality of
GLYMO can provide a cross-linking center for blending the
triblock copolymer and thus improving the compatibility
between organic and inorganic phases.
Figure 1 a shows the powder X-ray diffraction (XRD)
patterns of the hybrids with various [O]/[Li] ratios. The
appearance of peaks at 2 q = 0.58 indicates that the hybrids
exhibit mesoscopically ordered structures. The degree of
ordering depends on the lithium content. The much sharper
and more intense peak for the hybrid with [O]/[Li] = 16
indicates a higher degree of order than in the other two
samples. The small-angle X-ray scattering (SAXS) profile of
this hybrid (Figure 1 b) exhibits more and sharper peaks and
thus also indicates superior ordering in this sample. For [O]/
[Li]
pffiffi=
ffi 16,
pffiffiffi five scattering peaks with a spacing ratio of
1: 3:2: 7:3 are observed. This spacing sequence is indicative
of a hexagonal array of cylinders, with a cylinder spacing of
180 @.[10a] The TEM images in Figure 2 also show a wellorganized hexagonal mesophase with cylindrical assemblies
in the hybrid with [O]/[Li] = 16. Unlike mesoporous silica
materials, N2 adsorption–desorption isotherms (Supporting
Information) show that no significant adsorption is observed
for the sample after calcination at 560 8C for 8 h, because the
mesostructure collapses on removal of the copolymer. This
effect is due to the relatively low inorganic fraction (silica) in
these materials. Above its critical micellar concentration,
F127 forms micelles in water, in which a hydrophobic core
consisting of the PPO blocks is surrounded by an outer shell,
Figure 2. TEM images of the hybrid material with [O]/[Li] = 16 revealing
a well-ordered hexagonal array of cylinders, viewed a) from above and
b) from the side.
or corona, of hydrated PEO blocks.[10b] Consequently, the
silica species adopt the mesoscopic organization imposed by
the block copolymer and are preferentially partitioned into
the hydrophilic regions of the sample, where they form a rigid
cross-linked network. A schematic representation of the
mesophase formed by self-assembly of F127 is shown in
Scheme 1.
The glass transition temperature Tg and melting
temperature Tm of the hybrid samples were measured by
differential scanning calorimetry (DSC; Table 1). The Tg
of pure F127 copolymer is about 64 8C. The Tg value
increases (becomes less negative) with increasing lithium content, and this suggests that the presence of
lithium salts leads to the formation of transient crosslinks between ether oxygen atoms and lithium cations.
This cross-linking decreases segmental motion of the
polymer chains and thus increases Tg. Except for the
hybrid with [O]/[Li] = 32, no clear melting transitions
were observable, that is, F127 crystallization is successfully suppressed by high levels of salt doping. Thermogravimetric analysis shows that the present hybrid
electrolytes are thermally stable up to at least 200 8C.
Figure 3 shows the ionic conductivity s as a function
of
temperature
for hybrids with various [O]/[Li] ratios.
Figure 1. a) XRD and b) SAXS profiles of hybrids with various [O]/[Li] ratios.
The hybrid with [O]/[Li] = 16 has well-ordered mesoThe scattering wave vector Q is given in terms of Q = (4p/l) sin q, where 2 q
phases and exhibits the highest ionic conductivity,
is the scattering angle and l is the X-ray wavelength.
Angew. Chem. 2004, 116, 998 –998
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Zuschriften
Scheme 1. Schematic illustration of the formation of a mesophase by self-assembly of Pluronic F127 triblock copolymer. For the sake of clarity,
only the copolymer molecules and lithium cations at the cross-section of the cylinder assemblies are depicted, and the dissociated anions are not
shown. TEOS = tetraethoxysilane.
Table 1: DSC and ionic conductivity of hybrids with various [O]/[Li] ratios.
1
[O]/[Li]
Tg [8C]
Tm [8C]
Conductivity [S cm ]
at 30 8C
at 80 8C
32
16[a]
8
44.1
38.2
32.9
45.7
–
–
1.20 @ 106
3.17 @ 105
7.83 @ 106
1.58 @ 103
1.71 @ 103
7.24 @ 104
[a] The silica-free F127-based electrolyte at the same salt concentration
exhibits a conductivity of only 2.30 @ 107 S cm1 at 30 8C.
Figure 3. Temperature dependence of ionic conductivity of hybrids
with various [O]/[Li] ratios. For comparison, the conductivity of silicafree F127-based electrolyte with [O]/[Li] = 16 is also shown.
especially in the temperature range from 10 to 45 8C. An
optimal value of approximately 3 F 105 S cm1 at 30 8C is
obtained at [O]/[Li] = 16, which is comparable with the
conductivity of PEO-based electrolytes containing nanoscale
TiO2 and Al2O3.[11] This conductivity is much better than those
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
of composite polymer electrolytes containing other nanoscale
ceramics, and it is at least two orders of magnitude higher than
the conductivity of conventional PEO electrolytes. Interestingly, the conductivity of a silica-free F127-based electrolyte
of the same [O]/[Li] ratio, which does not have a mesoscopically ordered structure, is only 2.30 F 107 S cm1 at 30 8C. The
conductivity of the silica-containing electrolytes increases
with increasing temperature and reaches around 103 S cm1,
the level needed for many practical applications, at about
80 8C. The hybrid electrolyte with [O]/[Li] = 8 shows Vogel–
Tamman–Fulcher (VTF) behavior over the studied temperature range, that is, the ion mobility is coupled with the
segmental motion of the polymer chain, while the other two
hybrid electrolytes exhibit Arrhenius behavior between 10
and 40 8C. The hybrid with [O]/[Li] = 32 shows a conductivity
jump at the temperature associated with its melting point and
exhibits similar conductivity to the hybrid with [O]/[Li] = 16
at high temperatures. The hybrid with the largest amount of
lithium salt (i.e., [O]/[Li] = 8), which has less ordered
mesophases, exhibits the lowest ionic conductivity among
the studied hybrid electrolytes.
Solid-state 29Si and 13C NMR spectroscopy measurements
were performed in order to determine the structure of the
inorganic and organic parts of the hybrid, respectively. Three
major 29Si NMR signals at d = 58, 67, and 110 ppm are
observed, and can be assigned to silicon sites of T2 (RSi(OSi)2OH), T3 (RSi(OSi)3), and Q4 (Si(OSi)4) groups, respectively, where R represents an alkyl group belonging to
GLYMO. The 13C cross-polarization/magic-angle spinning
(CP/MAS) spectrum shows six major resonances, assigned to
CO linkages (d = 71, 74, and 76 ppm), CH2 groups (d =
24 ppm), CH3 (d = 18 ppm, PPO), and CSi (d = 10 ppm,
GLYMO). The relative intensities of T and Q and 13C signals
are all as expected on the basis of the synthesis. The absence
of the 13C peaks at d = 44 and 51 ppm for the epoxide ring of
GLYMO is indicative of complete epoxide ring opening.
Variable-temperature 7Li{1H} MAS NMR measurements
were performed to probe the local environments of the Li+
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Angew. Chem. 2004, 116, 998 –998
Angewandte
Chemie
cations. At 213 K, the 7Li resonance at d = 0.9 ppm (site I)
with a shoulder at about d = 0.4 ppm (site II) indicates that
two distinct 7Li local environments exist in the hybrid
(Figure 4). On raising the temperature of the sample to
Figure 4. Variable-temperature 7Li{1H} MAS NMR spectra, acquired at
a spinning speed of 3.4 kHz, of the hybrid with a [O]/[Li] ratio of 16.
233 K, which is close to Tg, these two sites merge together into
a signal at d = 0.6 ppm. Site I with the higher intensity peak
is assigned to lithium cations in the polyether domain,
whereas the site II possibly corresponds to the lithium cations
in the polymer/silica interface or silica-rich domain. Thus
variable-temperature 7Li{1H} MAS NMR spectroscopy is able
to resolve the different lithium local environments in the
hybrid system.
Previous NMR spectroscopic investigations on similar
hybrid materials showed that the hydrophilic PEO blocks are
firmly anchored in the inorganic silica phase, which substantially restricts the molecular mobility, while the hydrophobic
PPO blocks show a flexibility comparable to that observed for
the bulk copolymer.[12] Therefore, the hybrid electrolyte is
expected to exhibit low ionic conductivity, as the mobility of
the lithium cations is associated with the segmental motion of
the polymer chains. In contrast, the present hybrid electrolytes exhibit conductivity two orders-of-magnitude higher
than that of silica-free F127-based polymer electrolyte. The
resistive dynamics of polymer–salt electrolytes can be mediated by introducing nanoscale ceramics that function as solid
plasticizers.[11] In the present system the surface interaction of
the copolymer, cations, and anions with the nanoscale silica
network is believed to play a key role in stabilizing the hybrid
structure and facilitating motion of the Li+ ions while
retarding that of the anions. A possible explanation for the
lower conductivity of the hybrid with [O]/[Li] = 8 relative to
that with [O]/[Li] = 16 is increased ion-pair formation with
increasing salt concentration. However, FTIR spectroscopy
shows that the hybrids with [O]/[Li] = 16 and 8 exhibit similar
degrees of salt dissociation. The conductivity data in combination with the structural characterization suggests that the
Angew. Chem. 2004, 116, 998 –998
www.angewandte.de
drastically enhanced conductivity for the hybrid with
[O]/[Li] = 16 is closely related to its well-ordered mesophase,
which might improve the arrangement of Li+-ion conducting
pathways, as illustrated in Scheme 1. Both effects (i.e., surface
interaction and formation of a well-ordered mesophase) are
manifested as a substantial enhancement of the Li+ ionic
conductivity at room temperature. The ordered region of the
mesophase and the amorphous region of the polyether
moieties are possibly active simultaneously and work synergistically to yield favorable lithium-ion transport and thus
contribute to the high ionic conductivity of the materials.
In summary, we have demonstrated that the complexation
of Pluronic F127 triblock copolymer with LiClO4 by a sol–gel
route involving the co-condensation of alkoxysilanes can give
rise to a well-ordered mesophase that is dependent on the salt
concentration. The SAXS and TEM results confirm the
formation of stabilized and well-ordered hexagonal mesophase by self-assembly of F127 and lithium salts, especially at
a [O]/[Li] ratio of 16. The present self-organized nanocomposite network provides a novel architecture for ionically
conductive materials with a combination of several advantageous properties, for example, simple preparation from
commercially available components, substantial suppression
of polymer crystallization, reasonably high ionic conductivity
with copolymers of relatively low molecular weight, excellent
mechanical strength, and potential for nanostructurablility.
The present work can be extended to other similar triblock
copolymers which are capable of self-assembling on a silica
network, to make electrolytes with high ionic conductivity for
use in the field of lithium-battery technology.
Experimental Section
The organic–inorganic hybrid electrolyte based on F127 was synthesized by a sol–gel route. In a typical synthesis (3-glydicyloxypropyl)trimethoxysilane (1.5 g; GLYMO, Fluka) and tetraethoxysilane
(0.3 g; TEOS) were mixed and hydrolyzed with 0.01n HCl (30 mL)
at room temperature for 5–6 h. A solution of F127 (5.0 g) in
acetonitrile (30 mL) was mixed with LiClO4 to give the desired
[O]/[Li] ratio, and the mixture was stirred for 5–6 h. Then the two
solutions were mixed and stirred at room temperature for 2 days. The
resulting gels were dried at room temperature for 2 days and then
heated at 95 8C under vacuum for 2–5 days to give a piece of
transparent and crack-free material. Samples prepared with other
ratios of GLYMO/TEOS tended to be structurally unstable after
drying. They exhibited nonuniform shrinkage or had a constantly wet
surface. The salt concentrations were expressed as the ratio of the
total concentration of EO and PO ether oxygen atoms to the lithiumion concentration (i.e., [O]/[Li] ratio).
Powder XRD and SAXS data were collected on BL17A WigglerA beamline (l = 0.1326 nm) at National Synchrotron Radiation
Research Center of Taiwan. TEM images were taken with a Hitachi
H-7100 instrument operated at 75 keV. The samples were embedded
in Spur resin and cured at 60 8C overnight. Ultrathin sections (ca. 60–
90 nm) were cut from the embedded specimen with a diamond knife.
DSC studies were performed in the range of 60 to 125 8C by using a
Mettler Toledo DSC system at a heating rate of 5 8C min1. Alternating-current impedance measurements on the polymer electrolytes
were performed with a CH Instrument Model 604A Electrochemical
Analyzer over a frequency range of 10 Hz to 100 kHz with an
amplitude of 10 mV. All solid-state NMR experiments were performed on a Bruker AVANCE-400 spectrometer, equipped with a
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Zuschriften
Bruker double-tuned 7 mm probe. The Larmor frequencies for 7Li,
13
C, and 29Si nuclei were 155.45, 100.58, and 79.46 MHz, respectively.
A repetition time of 5 s was used for 13C CP/MAS NMR experiments.
A pulse length of 2 ms (p/6 pulse) and a repetition time of 50 s were
used to obtain 29Si MAS NMR spectra. Both 13C and 29Si chemical
shifts were referenced to external TMS at 0 ppm. To remove the
heteronuclear 7Li–1H dipolar interactions from the 7Li resonance,
high-power proton decoupling (7Li{1H} NMR) was used. 7Li chemical
shifts were referenced to external 1m aqueous LiCl at 0 ppm.
Received: June 30, 2003
Revised: September 9, 2003 [Z52243]
.
Keywords: block copolymers · conducting materials · lithium ·
organic–inorganic hybrid composites
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2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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